Method of producing metal nitrides

ABSTRACT

A method is provided for producing a selected metal nitride utilizing a salt bath. The selected metal is introduced into the salt bath in the presence of gaseous nitrogen and at least a certain amount of a halide of the selected metal. The salt bath is maintained at a temperature above its melting point for time sufficient to form a precipitate of the desired amount of a nitride of the selected metal. In accordance with a preferred embodment, the pressure is thereafter reduced to less than atmospheric and the temperature increased above the boiling point of the salt for a time sufficient to volatilize the molten salt which is removed to leave a precipitate of the selected metal nitride. The method is particularly applicable to the production of the mononitrides of uranium, plutonium, thorium, and mixtures thereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the manufacture of refractorycompounds. More particularly, the invention relates to the production ofthe nitrides of the refractory metals, rare earth metals, actinideseries metals, and combinations thereof. In its particularly preferredembodiment the invention provides a method for producing a mononitrideof thorium, uranium, plutonium, or mixtures thereof, which mononitrideis recovered in the form of ultrafine particles ideally suited forsintering to form a dense compact fuel element for a nuclear reactor.

2. Prior Art

It is well known that metals such as vanadium, titanium, thorium,niobium, zirconium, hafnium, tungsten, molybdenum, tantalum, uranium,plutonium, and silicon have refractory compounds including the carbides,nitrides, borides, phosphides, and the like. These compounds aredifficult to manufacture as their melting points are high, for example,in the region of 2000°-3000° C. The compounds have many purposes whichdepend upon their high melting points and chemical and physicalcharacteristics. The compounds of uranium, plutonium, and thorium are ofparticular interest in view of their potential use as nuclear fuelmaterials.

Indeed, the excellent physical and nuclear properties of the nitrides ofsuch materials mark them as an excellent fuel for use in a hightemperature, high power density nuclear reactor. In the case of uraniumnitride, for example, it can be substituted for UO₂ in a fuel elementand occupy about 30% less volume with an equivalent uranium content. Itshigh thermal conductivity (on a par with UC), high melting point, andacceptable chemical compatibility are further recommendations for itsuse as a nuclear fuel. However, the acceptance of this material as afuel for current commercial power reactors is at least partly contingentupon the overall economy associated with its production and fabricationinto a fuel element. One of the principal cost factors involved is thefabrication of the nitride into a densified compact. Specifically, toform a dense compact of the material, it is essential, if it is to beused as a fuel, that it be substantially pure and all in the mononitrideform, and additionally, have an ultrafine particle size.

Considerable work has been done in attempting to develop a viableprocess for the preparation of ultrafine particles of uraniummononitride. In U.S. Pat. No. 2,544,277 it is suggested that uraniumfirst be reacted with hydrogen until it is substantially all convertedto a uranium hydride. Thereafter, the uranium hydride is reacted withammonia or nitrogen at a temperature of from about 200° C. to 400° C. toform a uranium nitride, which subsequently is calcined at about 1400° C.to form a compound corresponding approximately to uranium mononitride.

In U.S. Pat. No. 3,180,702 it is suggested that uranium nitride beformed by reacting finely divided uranium particles with nitrogen in thepresence of hydrogen at a temperature between 450° C. and 1200° C.Thereafter, the excess nitrogen in the product uranium nitride isremoved by heating the uranium nitride in a vacuum at a temperatureabove about 1000° C. to form the mononitride.

U.S. Pat. No. 3,322,510 discloses yet another process for thepreparation of certain metallic nitrides. Prior to forming the nitride asurface hydride coating is provided on the selected metal by reacting itwith a small quantity of hydrogen, and thereafter the hydride-coatedmetal is reacted with nitrogen at elevated temperatures to form thenitride.

In East German Pat. No. 30,160 there is disclosed a process for theproduction of the nitrides of uranium and plutonium utilizing a moltensalt (alkali metal halide) bath. In the process disclosed therein, auranium halide (UCl₄) is reacted with ammonia to form a uranium nitrideand a byproduct of ammonium chloride. Thus, the process requires thatthe uranium metal first be reacted or treated to form a uranium halideand then introduced into the bath for reaction with the ammonia.Further, the process also results in an unneeded byproduct, i.e.,ammonium chloride.

SUMMARY OF THE INVENTION

The present invention provides a method of producing a selected metalnitride. The method comprises providing a molten salt bath in a reactionzone and introducing the selected metal into the molten salt bath in thepresence of nitrogen. There also is provided in the molten salt bath ahalide of the selected metal in an amount of at least 10 grams per literof molten salt. The molten salt bath is maintained at a temperature atwhich it is molten for a time sufficient to form a precipitate of adesired amount of a nitride of the selected metal. Thereafter the moltensalt bath is separated from the precipitated selected metal nitride, andthe selected metal nitride is recovered. In a preferred embodiment, themolten salt is removed by reducing the pressure in the reaction zone toless than atmospheric and increasing the temperature above the boilingpoint of the salt for a time sufficient to volatilize substantially allof the salt. SUch embodiment also facilitates the formation of the lowernitride of the selected metal. Specifically, under such conditionsexcess nitrogen is removed such that the product nitride is recovered,for example, as the mononitride.

The term "selected metal" as used herein, contemplates the refractorymetals, rare earth metals, and the actinide series metals. The term"refractory metals" as used herein, is defined as at least one metalselected from the group consisting of chromium, zirconium, titanium,niobium, tantalum, molybdenum, and tungsten. The rare earth metals arethose elements having an atomic number of from 57 thru 71, inclusive.The actinide series comprises elements having atomic numbers 89 thru103, inclusive. The particularly preferred metals for use in producing anitride in accordance with the claimed method are uranium, plutonium,thorium, and mixtures thereof. The present invention is particularlyuseful for the preparation of mixed mononitrides of uranium andplutonium for use as a fuel in a nuclear reactor. Titanium nitride andzirconium nitride are two nitrides which are readily prepared by thepresent method and are of particular value for their refractoryproperties combined with their high electrical conductivity andreasonable cost.

It is an advantage of the present invention that the selected metal neednot be ground to any particular fine size prior to nitriding. The sizeof the discrete portions of selected metal is not critical. Indeed, theselected metal may be introduced as a massive body. By a massive body itis meant, in contradistinction to such forms as granules, flakes, orpowders, a body having a depth or length along its shortest axis inexcess of about 1 cm. Thus, the present invention provides a methodwherein the selected metal may be introduced as fine granules, pelletsor even as massive chunks or in the form of bars, sheets, billets andthe like.

The term "salt bath" as used herein refers to an alkali or alkalineearth metal halide containing salt which is maintained at a temperatureabove its malting point during the nitriding operation. The molten saltmay be either a single alkali or alkaline earth metal halide or amixture of such halides, which may or may not be a eutectic mixture.

Typical examples of binary salt mixtures are sodium chloride-potassiumchloride, lithium chloride-potassium chloride, lithiumchloride-magnesium chloride, lithium chloride-sodium chloride, lithiumbromide-potassium bromide, lithium fluoride-rubidium fluoride, lithiumiodide-potassium iodide and mixtures thereof. Two binary salt eutecticmixtures having low melting points are lithium chloride-potassiumchloride (melting point 352° C.), and lithium bromide-rubidium bromide(melting point 278° C.).

Examples of ternary mixtures useful as the molten salt include calciumchloride-lithium chloride-potassium chloride, lithium chloride-potassiumchloride-magnesium chloride, lithium chloride-potassium chloride-sodiumchloride, calcium chloride-lithium chloride-sodium chloride, and lithiumbromide-sodium bromide-lithium chloride. Ternary eutectic mixtures withparticularly low melting points are lithium chloride-lithiumfluoride-lithium iodide (melting point 341° C.) and lithiumchloride-lithium iodide-potassium iodide (melting point 260° C.).

Although the ternary eutectic salt mixtures, paticularly thosecontaining the iodides, provide lower melting points, the binaryeutectic mixture of lithium chloride-potassium chloride sometimes ispreferred on the basis of its relatively low cost and relatively lowmelting point. The sodium chloride-potassium chloride eutectic generallyis preferred in spite of its higher melting point, since it is lower incost and more easily purified.

To form the desired nitride there obviously must be provided at least astoichiometric amount of free nitrogen. Generally there is provided asubstantial excess of nitrogen, since that which is not consumed in thereaction is readily recoverable. Nitrogen may be provided, for example,by introducing into the reaction zone either nitrogen gas or ammonia.The nitrogen preferably is provided by the introduction of gaseousnitrogen into the reaction zone. In accordance with the presentinvention, it is not essential that the nitrogen be introduced into themolten salt. For example, it has been found that the method workseffectively simply by providing an appropriate amount of gaseousnitrogen as a cover gas over the molten salt. Generally, however, it ispreferred to bubble the gas upwardly through the molten salt, as suchmethod of introduction greatly enhances the reaction rate, thus reducingthe time required.

It is an essential feature of the present invention that there beprovided in the molten salt bath a corresponding halide of the selectedmetal in an amount of at least about 10 grams per liter of molten saltbath. Generally, the selected metal halide is provided in an amount offrom about 10 to 200 gms/liter and particularly good results areobtained when it is present in an amount of from 100 to about 200gms/liter.

Preferably, the selected metal halide and the molten salt bath arechosen such that the selected metal halide will have a solubility in thebath within the range of from about 10 to 200 grams per liter. In suchinstances the selected metal halide may be introduced directly into themolten salt bath. Alternatively, a halide gas is introduced into themolten salt bath for reaction with the selected metal to form thenecessary amount of selected metal halide. In yet another variant, theselected metal halide in vapor form, is introduced concurrently with thenitrogen, thus providing the required amount. Any halide of the selectedmetal may be used, however, chloride generally is preferred based uponits lower cost.

The exact mechanism by which the present reaction takes place is notknown with certainty, and the inventor does not wish to be bound by anyparticular theory. It is hypothesized that the selected metal, forexample, uranium, serves as the uranium source in making the nitride inaccordance with the following equations:

U+ 3ucl_(UCl) ₄ → 4UCl₃

4UCl.sub. 3 + 1/2 N₂ →UN+ UCl₄

Net Reaction= U+ 1/2 N₂ → U_(n)

It is seen that the uranium halides are regenerated and that onlyuranium and nitrogen are consumed. It must be appreciated, however, thatthis is only a hypothesis as to the mechanism of the reaction. Indeed,it may be that the uranium halide or the halide in combination with themolten salt act in some manner as a catalyst, rather than the halideacting as an intermediate in the reaction. It is known with certainty,however, that only the selected metal and nitrogen are consumed in theproduction of the selected metal nitride. Therefore, unlike the priorart processes, it is not necessary to utilize hydrogen to prepare themetal for nitriding, nor are any unnecessary byproducts formed.

it is another advantage of the present invention that the reaction willoccur at relatively low temperatures. Specifically, it is not necessaryto exceed the melting point of the selected metal nitride product oreven the melting point of the selected metal. Generally, the temperatureis limited on the low side only by the melting point of the saltselected for the bath. Depending upon the selection of the particularsalt bath, such temperature may be as low as about 250° C. The lithiumchloride-potassium chloride eutectic, for example, melts at atemperature as low as about 352° C., and the presence of the selectedmetal halide may further reduce the melting temperature.

The upper temperature is limited by the boiling point of the selectedmolten salt. Specifically, if the temperature is so high as tovolatilize the molten salt bath, obviously the benefits of the bathwould be lost. Generally, it has been found that a temperature of fromabout 250° to 900° C. and preferably a temperature of from 350° to 800°C. are sufficient for the formation of the selected metal nitride.Pressure is not critical and substantially any pressure may be utilized,subject, of course, to its effect on the volatilization of the alkalimetal halide bath.

The method should be practiced in a closed reaction zone under asubstantially inert atmosphere, since many of the selected metals,including their halides or nitrides will react with atmosphericconstituents such as oxygen and water vapor and become contaminated.This is particularly true in the case of the particularly preferredselected metals, i.e., uranium, plutonium, and thorium. The inertatmosphere generally is provided by the nitrogen gas, although, ifdesired, other non-reactive gases could be used to provide such an inertatmosphere, such as for example, helium, argon, and the like.

The time required to effect the complete reaction of all the selectedmetal to the desired nitride product will vary, depending upon suchthings as the manner in which the nitrogen gas is introduced, thereaction temperature, as well as the size and configuration of thediscrete portions of selected metal introduced into the reaction zoneand the concentration of the selected metal halide. The appropriate timefor any given set of conditions is readily determinable by those skilledin the art through routine experimentation.

Still another advantage of the present invention is that the productnitride is precipitated in the form of ultrafine particles. The nitrideprecipitate is found to have a median particle size of less than about 5microns, generally from 0.1 to 1.0 microns. Thus, the product is readilyamenable to compaction and sintering for use as a fuel element in thecase of plutonium, uraniun, thorium and combinations thereof. Indeed,utilizing the product nitride of the present invention, it is possibleto form a compact sinter having a density in excess of 90% of that ofthe nitride.

In accordance with a preferred embodiment of the present invention,after a desired amount of the nitride of the selected metal has beenprecipitated, the pressure in the reaction zone is reduced to belowatmospheric and the temperature increased to above the boiling point ofthe selected salt whereby the salt is volatilized to facilitate itsseparation from the precipitated product. A pressure range of from about1 to 100 mm/Hg generally will suffice with a pressure of less than 10mm/Hg being preferred. The salt generally is selected to have a boilingpoint such that it is readily volatilized at a temperature of from about600° to 1500° C. (preferably 600° to 1000° C.) at the reduced pressure,and the salt is maintained at such temperature for a time sufficient tovolatilize substantially all the salt which is removed from the reactionzone. Such procedure has the added advantage of also moving any excessnitrogen from the selected metal nitride. Thus, the remaining productmetal nitride is recoverable as the more dense and thermodynamicallystable monoitride. After removal of the volatilized salt from thereaction zone, the selected metal nitride then is readily recoverable ina substantially pure ultrafine particle form.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be more clearly understood by reference to thedescription below taken in connection with the accompanying drawingwherein the sole FIGURE is a diagrammatic view in section of anapparatus which may be used in carrying out the method of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In order that the description may be presented in a more concrete formit will be assumed hereinafter for the sake of convenience that themethod is applied to the nitriding of a body of uranium.

Referring to the drawing therein is depicted an oven or furnacedesignated generally by the reference numeral 10, which comprises a bodyof insulation 12, surrounded by a metal cover 14 and including a heatingmeans such as an electrical heating coil 16. The reaction zone isdefined by an elongated metal housing 18, which is located withinfurnace 10. Contained within the reaction zone is a bar of uranium metal20 and a body of salt 22. Metal housing 18 is closed by a cover plate24, which is in sealing engagement with housing 18 and retained by aplurality of fasteners such as bolts 26. Communication into the interiorof the reaction zone is provided by tube 28, which extends down into thebody of salt and tube 30 which opens into the upper portion of thereaction zone. Tubes 28 and 30 are provided with valves 34 and 36,respectively, for isolating the reaction zone from the exterior. Meansalso are provided for monitoring the pressure and temperature within thereaction zone, such as pressure gage 32 and temperature sensor andindicator 38.

To demonstrate the efficacy of the present method, the followingspecific example is performed under adverse conditions. A salt mixturecomprising 9 gms sodium chloride, 11 gms potassium chloride and 4 gmsuranium tetrachloride are placed in housing 18. Also placed in thehousing are two bars of uranium metal, 1/4 in.× 1/8 in.× 2 in. (totalweight 38.4 gms). The cover is secured in place and the reaction zoneevacuated through tube 30 and valve 36 to a pressure of about 10⁻ ³mm/Hg. Thereafter, valve 36 is closed and furnace 10 is brought to atemperature above the melting point of the salt to form a molten bath.After the temperature has stabilized at about 700° C., a measured amountof nitrogen sufficient to raise the pressure in the reaction zone toabout 0.9 atmospheres is introduced through valve 36 and tube 30,providing a blanket of nitrogen above the molten salt bath.

As the nitriding reaction takes place, it depletes the amount ofnitrogen in the reaction zone, resulting in a gradual pressure drop. Thegradual pressure drop is used as a measure of the nitrogen absorbed. Atthe end of eight hours it is calculated that the reaction has consumed1000 cc of nitrogen, which would be equivalent to the formation of 22.5gms of uranium mononitride. At the end of this time the nitrogen ispumped out and the temperature is increased to 900° C. for 30 minutes tovolatilize the salt. Valve 36 is opened and the volatilized salt removedfrom the reaction zone.

The uranium metal then is removed, cleaned, dried, and weighed, showinga loss of 19.8 gms, which is equivalent to 20.9 gms of uraniummononitride. The nitriding product is recovered as a fine black powderand is analyzed by x-ray difraction. Predominant lines formed a verysharp pattern for uranium mononitride. The weight of the product powderrecovered was 19.2 gms, however, some small quantity had been lost inremoving it from the reaction zone. Thus, the three measurements of thequantity of the UN produced are all in substantial agreement within thelimitations of the test method utilized.

The black power product has very fine granular size and exhibited strongpyrophoric behavior which is indicative of ultrafine uraniummononitride. A portion of the product is examined under an electronmicroscope. The particles appear to have a median size of substantiallyless than 1 micron and indeed, many discrete particles appear as smallas 0.1 microns in diameter.

It is seen that even under the most adverse conditions, i.e., when themetal is introduced as a massive body and the nitrogen gas is introducedabove the bed of molten salt, the present method still produces uraniummononitride as an ultrafine particle particularly well-suited forcompaction and sintering for use as a fuel element in a nuclear reactor.It will be appreciated that the rate of reaction would be greatlyenhanced if the metal were introduced as smaller chips or chunks and thenitrogen gas were bubbled through the molten salt, i.e., by introductionthrough tube 28 and valve 34, rather than tube 30 as previouslydescribed.

It will be understood that the invention is not limited in any sense tothe form of embodiment which has been described and illustrated, butrather is intended to include within its scope the various alternativeforms which will be apparent to those skilled in the art.

What is claimed is:
 1. A method of producing a selected nitridecomprising the steps of:a. providing a salt bath in a reaction zone; b.introducing the selected metal into the salt bath in the presence ofgaseous nitrogen; c. providing a halide of the selected metal in anamount of at least 10 gm per liter of salt; d. maintaining the salt bathat a temperature above its melting point and below its boiling point fora time sufficient for a desired amount of the selected metal to bereacted and form a precipitate of a nitride of the selected metal; e.removing the salt from the reaction zone; and f. recovering the selectedmetal nitride.
 2. The method of claim 1 wherein the salt is removed fromthe reaction zone by reducing the pressure in the reaction zone to fromabout 1 to 100 mm/Hg and increasing the temperature above the boilingpoint of the salt for a time sufficient to volatilize the salt and thevolatilized salt is removed from the reaction zone.
 3. The method ofclaim 1 wherein the gaseous nitrogen of step (b) is provided byintroducing gaseous nitrogen into the lower portion of the salt bathwhile the salt is maintained in a molten state.
 4. The method of claim 1wherein the selected metal halide is provided by introducing anelemental halide gas into the reaction zone for reaction with theselected metal.
 5. The method of claim 1 wherein the salt comprises analkali metal chloride.
 6. The method of claim 1 wherein such selectedmetal is selected from the actinide series of elements having an atomicnumber of from 89 thru 103, inclusive.
 7. A method of producing aselected metal nitride comprising the steps of:a. providing a moltensalt bath in an inert atmosphere in a closed reaction zone; b.introducing the selected metal into the molten salt bath in the presenceof gaseous nitrogen, said selected metal being selected from the groupconsisting of plutonium uranium, thorium, and mixtures thereof; c.providing a halide of the selected metal in an amount of at least 10 gmsper liter of molten salt; d. maintaining the molten salt bath at atemperature above its melting point and below its boiling point, saidtemperature being within the range of from 250° to 900° C, for a timesufficient for a desired amount of the selected metal to be reacted toform a precipitate of a nitride of the selected metal; e. reducing thepressure in the reaction zone to less than about 10 mm/Hg and increasingthe temperature above the boiling point of the salt, said temperaturebeing within the range of from about 500° to 1500° C, for a timesufficient to volatilize the molten salt; f. removing the volatilizedsalt from the reaction zone; and g. recovering the selected metalmononitride as a fine precipitate having a particle size of less than 5microns.
 8. The method of claim 7 wherein the gaseous nitrogen of step(b) is provided by introducing gaseous nitrogen into a lower portion ofthe salt bath while it is maintained in a molten state.
 9. The method ofclaim 7 wherein the selected metal halide is provided by introducing anelemental halide gas into the reaction zone for reaction with theselected metal.